Recovery of Phosphate From Aqueous Solution by Mg/Ca-Modified Biochar Derived From Dewatered Dry Sludge and Waste Almond Shells and Its Potential Application

Phosphorus is an essential element of ecosystems, supporting the growth of plants and animals; however, its excessive presence in water can lead to eutrophication. In this study, the phosphate adsorbent SA2:8 Mg/Ca was prepared by loading calcium and magnesium onto sludge and almond shell. Various characterization methods were used to analyze biochar, and the phosphorus removal effect of SA2:8 Mg/Ca under different conditions was evaluated. The results showed that the phosphate removal rate of the adsorbent was more than 70% in the range of pH 4 ~ 12, and the adsorption performance was significantly affected by HCO₃⁻ and SO₄²⁻, though the removal rate remained over 50%. The adsorption process conforms to the pseudo-second-order model, and the isothermal adsorption model aligns more closely with the Langmuir model. Increased temperature was favorable for phosphate adsorption. The theoretical maximum adsorption capacity of SA2:8 Mg/Ca was 78.27 mg/g. Thermodynamic analysis confirmed that the adsorption process was spontaneous. The main mechanisms of adsorption include electrostatic interaction, ion exchange, Lewis acid–base interaction, and chemical precipitation. Additionally, the P-laden biochar exhibited excellent potential for application: it can be used as a catalyst to improve the efficiency of persulfate catalytic degradation of ofloxacin. The removal rate of OFX in water by the SA2:8 Mg/Ca-PS system was 81.4%.

Graphical Abstract